This invention relates generally to techniques of optically measuring temperature.
Optical techniques for measuring the temperature of surfaces or other solid objects, and fluids or other environments, are becoming increasingly utilized in place of more traditional electrical techniques, such as those utilizing thermocouples, thermistors or resistance thermometry devices (RTDs). An early optical technique for measuring the temperature of surfaces was infrared radiometry. With this technique, the infrared energy being emitted from the surface of interest was measured by a non-contact technique. If the emissivity of the surface is known, its temperature can be calculated from the infrared emission intensity measurement. Since it is often difficult to know with precision the emissivity of the surface of interest, such measurements are not always made with the accuracy that is desired. Further, infrared radiometry is a technique which is made useful for measuring high temperatures but is not as useful for applications requiring measurement of low temperatures.
Various other optical techniques have been suggested for measuring lower temperatures. One such technique utilizes a liquid crystal film enclosed in a housing at a tip of an optical fiber probe designed for implantation in biological tissue to measure its temperature. The proportion of light directed against the sensor which is reflected by it is an indication of temperature. An example of this technique is given in U.S. Pat. No. 4,016,761 --Rozzell et al. (1977). Another technique, described in U.S. Pat. No. 4,140,393 --Cetas (1979) utilizes a birefringement crystal as the temperature sensing element at the end of an optical fiber. U.S. Pat. No. 4,136,566 --Christensen (1979) relies upon a shifting light absorption edge of a semiconductor material as a function of temperature.
Many other types of optical temperature sensors have been proposed but the use of a photoluminescent sensor has found the widest commercial acceptance for lower temperatures. Early photoluminescent devices continuously excited the sensor to luminescence and measured the relative intensities of the resulting emission in defined wavelength bands. Implementations of this technique are described in U.S. Pat. Nos. 4,448,547 --Wickersheim (1984) and 4,376,890 --Engstrom et al. (1983).
More recently, the temperature dependent decay time of photoluminescence is utilized in temperature measuring instruments. The sensor is excited to luminesce by directing against it a time varying excitation radiation signal and a time varying characteristic of the resulting luminescence is detected and processed to extract temperature information from it. Examples of this are given in U.S. Pat. Nos. Re.31,832 --Samulski (1985) and 4,652,143 --Wickersheim et al. (1987), and U.K. Patent No. 2,113,837B --Bosselmann (1986). The commercial forms of such products form a sensor of the photoluminescent material at the end of a single optical fiber. Because the technique measures temperature dependent decay time changes in luminescent intensity, rather than absolute levels, the systems require little or no calibration in order to provide measurements of acceptable accuracy.
The photoluminescent techniques are particularly useful for measuring from low temperatures (such as -100.degree. or -200.degree. C.) to moderately high temperatures (300.degree. to 500.degree. C.). However, since the technology depends on the phenomenon of thermal quenching of luminescence, the sensor materials cease emitting light at very high temperatures. There are a very small number of photoluminescent materials which can be used up to 1000.degree. C. or so but these materials have a very limited range of use and cannot be used at much lower temperatures.
As a separate body of technology, fiber optic probe temperature sensors have also been developed utilizing black body structures as sensors. Examples of this technology are given in U.S. Pat. Nos. 4,576,486 --Dils (1986), 4,750,139 --Dils (1988), and 4,845,647 --Dils et al. (1989). Designed primarily for measuring extremely high temperatures, an optically transmitting probe that can withstand those temperatures is coated at one end with an appropriate opaque material to form a black body cavity. Temperature dependent infrared emission from the black body cavity is carried along the optical transmission medium to a connecting optical fiber and then to a measuring instrument. Alternatively, an external object or surface can be made into the shape of a black body cavity and a light pipe used to gather, with or without use of a lens system, its emissions for transmission to a detector without contacting the black body.
While such an infrared system, when used with appropriate near infrared detectors and transmitting materials, can also cover a wide temperature range, it does not work well at lower temperatures (e.g. below about 200.degree. C. to 300.degree. C.) and thus cannot be used down to most ambient temperatures. In order to remedy this low temperature limitation to some extent, an electrical technology, such as one using thermocouples, is sometimes used in combination with the black body sensor, an awkward combination of optical and non-optical temperature measurement technologies.
Furthermore, while the black body emission follows very well defined laws of physics in terms of its dependence on temperature, the emission is modified by other factors, such as losses in connectors and the transmitting materials. Since the system depends on an intensity measurement, calibration of the complete system is required. This is not always convenient or possible, especially in industrial process control or aerospace applications, where the optical transmission cables are built into the system in advance and the sensors may be quite remote and typically inaccessible.
It is a principal object of the present invention to provide an optical system for accurately measuring temperatures over a very broad temperature range. It is another object of the present invention to provide such a system which is essentially self-calibrating.